1. Field of the Invention
The present invention refers to a circuit apparatus with LED diodes.
2. Description of the Related Art
Liquid crystal displays are widely used in mobile telephones; said displays use a large number of LED diodes to permit the phenomenon of backlighting. The LED diodes are distributed in the displays uniformly and use the same bias current; to obtain this they are connected in series.
To feed chains of serially connected LED diodes for emission of white light, devices suitable for increasing the feed voltage above the value of the feed voltage at their input are employed.
The most adopted circuit solutions provide for the use of a boost converter which, supply many branches connected in parallel and each one made up of a series of LED diodes, permit the setting of the current or the voltage on each one.
To regulate the current that passes through one or more branches of LED diodes there are two different modes: a current one and a voltage one. In both methods all the branches supplied by the boost converter are connected in parallel.
In the first mode only the current of the main branch can be set. The output current is read and compared with a reference to generate a control in pulse width modulation (PWM) mode; the circuit branches that are not controlled directly can even have a current very different from that of the main branch.
The disadvantage lies in the parallel connection of the circuit branches. Even if the current that flows in the main branch with the highest number of diodes is controlled directly, the secondary circuit branches can have an additional voltage and a different current. Adding a series of resistances in the secondary branches the current set on the main branch can be reached seeing that the resistances compensate the voltage jump error between the main branch and the secondaries that is due to the connection in parallel. In any case even if the object is reached a consistent quantity of power dissipation (on the compensation resistances) causes the decrease in the efficiency of the control.
This disadvantage can be present not only when supply the circuit branches with a different number of diodes, but also if the number of LED diodes is equal in all the branches. In fact the voltage jump between the LED diodes could be different even if the same current flows. As a consequence it is necessary to impose a different voltage jump for each branch, but this is not possible by connecting all the branches in parallel. Only by regulating the current that flows through the circuit branches with a maximum value of voltage jump and inserting variable resistances in the other circuit branches the parallel connection can be maintained.
Another problem lies nevertheless in the method of identifying the circuit branch with the highest voltage jump by adjusting the other branches with resistances and then adding power consumption.